1. Field of the Invention
The present invention relates to a noise control device, and more particularly to a noise control device which, even when a noise includes a frequency band for which a noise control process cannot be performed within a noise transfer time because of downsizing, cost reduction, and the like, reduces the noise in a frequency band for which the noise control process can be performed within the noise transfer time, while preventing an adverse effect in the frequency band for which the noise control process cannot be performed within the noise transfer time.
2. Description of the Background Art
There has been, for a long time, an idea of the so-called active noise control, in which a sound having the opposite phase to the phase of a noise outputted from a noise source is reproduced from a control speaker to thereby cancel the noise. Conventionally, a practical use of the active noise control using an analog feedback control (hereinafter referred to as a FB control) has been pursued, and these days, the analog FB control is generally used in a headphone and the like. FIG. 45 shows a fundamental configuration of a noise control device using the FB control.
When an analog FB control is used, costs can be suppressed to be relatively low. However, an analog FB control has a problem that it is difficult to realize complicated control characteristics, and moreover a problem that it is difficult to obtain a stable and excellent noise reduction effect because of an oscillation condition involved in the FB control. Even though there are the problems mentioned above, an analog FB control in a one-dimensional space, such as for a headphone, is an appropriate choice in consideration of cost performance, and in fact, there are a number of examples of the practical use thereof.
However, when the FB control using a digital control is performed in a three-dimensional space such as an automobile, a process for the FB control becomes more complicated than for a feed forward control (hereinafter referred to as an FF control), due to oscillation and the like. Accordingly, there are very few examples of a practical use in which the FB control is performed using the digital control. In addition, even in a one-dimensional control such as for a ventilation duct, a digital adaptive FF control is dominant, in consideration of a noise change and a secular change of control speaker characteristics, microphone characteristics, and the like. Therefore, the FF control will firstly be described.
FIG. 43 shows a fundamental configuration of the FF control.
A signal processor 300 processes a noise signal and generates a control signal, in time for a noise from a noise source 1 reaching a control point 4 via a noise transfer system 200. Then, at the control point 4, the signal processor 300 applies the control signal to the noise that reaches the control point 4 via the noise transfer system 200. As a result of a synthesis of the control signal and the noise at the control point 4, the noise is reduced. That is, the signal processor 300 may generate a control signal having the opposite characteristics (the same amplitude and the opposite phase) to the characteristics of the noise that reaches the control point 4 via the noise transfer system 200. In addition, as clear from FIG. 43, in the FF control, a noise transfer time T in the noise transfer system 200 and a processing time τ in the signal processor 300 have to satisfy the relationship of τ=T. Here, when the FF control shown in FIG. 43 is realized using a digital process, the signal processor 300 is normally formed with a digital filter such as an FIR filter. Therefore, the process performed by the signal processor 300 inevitably becomes a time-delay process due to a digital delay. Accordingly, since a delay time can be finely adjusted by a coefficient of the FIR filter for example, a condition for the processing time in the signal processor 300 may be τ≦T. Thus, a noise control process can be performed within a noise transfer time.
FIG. 43 is on the assumption that the noise transfer system 200 does not vary, but in fact, the noise transfer system 200 often varies. For example, in a case where the noise transfer system 200 is applied to an exhaust pipe or the like: an acoustic velocity changes depending on a temperature; and, in a noise of a running automobile, a noise (transfer characteristics) changes depending on a running state such as a road surface and a speed, or the number of passengers and positions of the passengers, and the like. In order to absorb these changes, an adaptive FF control is performed. FIG. 44 shows a fundamental configuration of a noise control device in a case where the adaptive FF control is used. As shown in FIG. 44, a result in the control point 4 is, as an error signal, returned to the signal processor 300, and the signal processor 300 changes control characteristics (a coefficient) of the signal processor 300, based on the error signal.
Here, when the adaptive FF control shown in FIG. 44 is used, a correlativity (coherence) between a reference signal (a signal a of FIG. 44), which is a noise signal outputted from the noise source 1, and the error signal (a signal b of FIG. 44) is important, for an accurate convergence of the control characteristics (a coefficient) of the signal processor 300. When the correlativity is low, an accurate coefficient cannot be obtained, and as a result, a sufficient noise reduction effect cannot be obtained (for example, Japanese Laid-Open Patent Publication No. 5-52645; and ACTIVE CONTROL OF SOUND, P. A. Nelson & S. J. Elliott, ACADEMIC PRESS, P177). Particularly, Japanese Laid-Open Patent Publication No. 5-52645 also mentions a multiple coherence in a case of a three-dimensional control, and shows that, in this case as well, the higher the coherence is, the larger the obtained noise reduction effect becomes.
From the above, it can be said that, in the adaptive FF control, conditions for obtaining the maximum noise reduction effect are that:
(1) the noise control process can be performed within the noise transfer time; and
(2) the correlativity between the reference signal and the error signal is high.
In order that the noise control process can be performed within the noise transfer time, it is necessary that, in FIGS. 43 and 44, the processing time τ in the signal processor 300 is shorter than the noise transfer time T, which however cannot always be satisfied because of the problem of costs and the like. For example, as a method for shortening the processing time τ in the digital control, increasing a sampling frequency can be mentioned firstly. In this case, on the contrary, a problem occurs that a time usable for a signal process is shortened (because a time usable as the processing time is given as the inverse of the sampling frequency) and thus a sufficient amount of operations cannot be ensured, and the like. Consequently, it is necessary to reduce the total amount of operations such that the operations can be performed within the processing time by reducing the number of control filter taps in the signal processor 300 or, in a three-dimensional control, by reducing the number of noise processes (=the number of control filters). As a result, a noise reduction effect cannot be obtained in a low frequency range due to a lack of the number of taps, or an efficient noise reduction effect cannot be obtained because the multiple coherence is lowered due to a lack of the number of noise processes.
On the other hand, when the sampling frequency is lowered for ensuring the time for operations, the processing time τ in the signal processor 300 is increased, and it becomes necessary to increase the noise transfer time T by the amount of the increase of the processing time τ in order that the noise control process can be performed within the noise transfer time. This normally means increasing a length from the noise source 1 to the control point 4. As a result, a noise control system as a whole is increased in size, and a problem may occur that this noise control system cannot be applied to a small-size product such as headphones and vacuum cleaners. Moreover, actually, when the length from the noise source 1 to the control point 4 is increased, the correlativity between the reference signal and the error signal is often lowered. For example, in a case of a ventilation duct, a noise of a fan which is a noise source, and the like, transfers within the duct, and exhaust air as a fluid also passes together with the noise. Therefore, a turbulent flow or the like is generated in a region between the noise source 1 to the control point 4, to lower the correlativity. Thus, the longer the distance from the noise source 1 to the control point 4 is, the more likely the turbulent flow is to occur to lower the correlativity. In another example, in a noise of a running automobile, train, and the like, many noises, such as not only an engine noise and a motor noise but also a road noise, a wind noise, noises of surrounding vehicles, enter the inside of a vehicle. Therefore, it is difficult to ensure that, for all noises, noise signals as reference signals are detected at noise sources which are originating points of the noises. Consequently, the noise signal is detected in the middle of a noise transfer system. In such a case, as a point at which the noise signal is detected is nearer the control point 4, the correlativity between a reference signal and an error signal becomes higher. In other words, the longer the distance from the noise source 1 to the control point 4 is, the lower the correlativity becomes.
In this manner, in a normal noise environment, shortening a time for the noise control process and increasing the correlativity between the reference signal and the error signal are incompatible with each other. Therefore, conventionally, they are balanced with each other, for the practical use.
Here, an influence in a case where the noise control process cannot be performed within the noise transfer time will be described in more detail.
FIG. 46 shows a control coefficient and a noise reduction effect in a case where the signal processor 300 shown in FIG. 43 or FIG. 44 can perform the noise control process within the noise transfer time. In FIG. 46: (1) shows impulse characteristics of the control coefficient (when an FIR filter having 2048 taps is used, for example) of the signal processor 300; (2) shows, in its upper section, noise characteristics before a control (control “OFF”) and noise characteristics after the control (control “ON”); and (2) shows, in its lower section, control OFF-ON difference characteristics, that is, the amount of the noise reduction effect. In FIG. 46, since the noise control process can be performed within the noise transfer time; in the control coefficient, a peak of an impulse is expressed in a good manner within coefficient taps, and also the noise reduction effect of approximately 60 dB can be obtained for all the frequencies.
On the other hand, FIG. 47 shows a control coefficient and a noise reduction effect in a case where the signal processor 300 shown in FIG. 43 or FIG. 44 cannot perform the noise control process within the noise transfer time. In impulse characteristics shown in (1), a peak of an impulse is not placed within coefficient taps but is beyond the 0th tap. That is, the fact that the noise control process cannot be performed within the noise transfer time is expressed as the control coefficient. Moreover, in the noise reduction effect of (2), a noise is not reduced at all for all the frequencies. In this manner, in the case where the noise control process cannot be performed within the noise transfer time, a problem occurs that the noise reduction effect cannot be obtained. Here, FIG. 47 shows a condition in a case where the noise control process cannot be performed within the noise transfer time for all the frequencies. However, actually, the noise control process often cannot be performed within the noise transfer time only for a certain frequency band. This will be indicated below.
FIG. 48 is a re-description of FIG. 43, showing a configuration similar to an actual example in which an analog is mixed. In FIG. 48, an AD (analog-digital) converter 5, a DA (digital-analog) converter 6, analog LPFs (low pass filters) 7 and 8 for anti-aliasing are added before and after the signal processor 300. Here, when a delay (the same value in all the frequencies) of the AD converter 5 is defined as τ1, a delay (the same value in all the frequencies) of the DA converter 6 is defined as τ2, and each of delays (maximum group delays) of the LPFs 7 and 8 is defined as τ3,τ+τ1+τ2+2×τ3≦T has to be satisfied in order to perform the noise control process within the noise transfer time. Here, FIG. 49 shows characteristics of the analog LPF of FIG. 48. When the LPFs 7 and 8 have the characteristics shown in FIG. 49, the maximum group delay τ3 is equal to or greater than 30 samples around 10 kHz (30/48000=0.625 msec, when a sampling frequency is 48 kHz). Since each of the values of τ, τ1, and τ2 is the same in all the frequencies, the point of the control is whether or not the noise control process can be performed within the noise transfer time at a frequency of around 10 kHz which corresponds to the maximum group delay τ3 of the LPFs 7 and 8.
FIG. 50 shows (1) a control coefficient and (2) a noise reduction effect in a case where the noise control process cannot be performed within the noise transfer time in FIG. 49. As seen from FIG. 50(2), the amount of reduced noise is 20 to 30 dB in a low frequency range, but the effect deteriorates in a higher range, and conversely the noise increases at a frequency around 10 kHz which corresponds to the maximum group delay τ3 of the LPFs 7 and 8. Also in FIG. 50(1), a peak of impulse characteristics of the coefficient is not placed within coefficient taps.
For reference, FIG. 51 shows (1) a control coefficient and (2) a noise reduction effect in a case where the noise control process can be performed within the noise transfer time in FIG. 49. Referring to the noise reduction effect in FIG. 51(2), the amount of reduced noise becomes small at a frequency equal to or higher than 10 kHz, but unlike FIG. 50(2), the noise does not increase. Also, in the control coefficient in FIG. 51(1), a peak of impulse characteristics of the coefficient is placed within coefficient taps, and the noise control process can be performed within the noise transfer time. In FIG. 51(2), the amount of reduced noise becomes small at a frequency equal to or higher than 10 kHz, and this is because the level of the LPFs 7 and 8 is drastically lowered as shown in FIG. 49(1). Since the signal processor 300 receives an influence thereof, the amount of reduced noise becomes small at a frequency equal to or higher than 10 kHz. If the noise control process can be performed within the noise transfer time, the noise is not increased but can be reduced.
In the above, the decrease of the amount of reduced noise and the noise increase in a high frequency range, due to the influence of a time for the noise control process, have been described. However, in a low frequency range as well, a decrease of the amount of reduced noise and a noise increase may occur. In a low frequency range, a main factor of a delay of the noise control process is a group delay of a control speaker. FIG. 52 shows characteristics of a general speaker. A resonant frequency fo of the speaker is approximately 150 Hz. Referring to group delay characteristics of FIG. 52(3), the speaker exhibits a group delay of 2 msec at the resonant frequency fo. The speaker exhibits a larger group delay at a frequency equal to or lower than the resonant frequency fo, but exhibits a smaller group delay at a frequency equal to or higher than the resonant frequency fo.
Similarly to the description of the amount of reduced noise and the noise increase in a high frequency range, the amount of reduced noise and a noise increase in a low frequency range will be described, by using a HPF (high pass filter) to realize the speaker characteristics. FIG. 53 is a diagram showing HPFs being additionally inserted to an output of the signal processor shown in FIG. 43. FIG. 54 shows amplitude characteristics and group delay characteristics of first-order HPFs 9 and 10 shown in FIG. 53. Here, in FIG. 52(1), a level of the speaker in a low frequency range drops at −12 dB/oct. Therefore, by using two first-order HPFs having the same cutoff frequency fc=150 Hz and the same cutoff characteristics of −6 dB/oct., the characteristics shown in FIG. 52 are approximated. As shown in FIG. 54(2), a group delay of each of the first-order HPFs 9 and 10 at 150 Hz is 25 samples (25/48000=0.521 msec). Accordingly, a group delay obtained by using the two first-order HPFs 9 and 10 is approximately 1 msec. The group delay of the first-order HPFs 9 and 10 of FIG. 54 is smaller than the group delay of the speaker of FIG. 52. However, in a case where the noise control process cannot be performed within the noise transfer time, the noise reduction effect deteriorates in a lower range, and the noise increases at a frequency equal to or lower than 100 Hz, as shown in FIG. 55(2). That is, the same situation as the deterioration of the noise reduction effect and the noise increase in a high frequency range occurs in a low frequency range, too, due to the group delay of the speaker. For reference, FIG. 56 shows (1) a control coefficient and (2) a noise reduction effect in a case where the noise control process can be performed within the noise transfer time in FIG. 53. Referring to the noise reduction effect in FIG. 56(2), similarly to in a high frequency range, the amount of reduced noise becomes smaller in a lower range, but unlike in FIG. 55(2), the noise does not increase, if the noise control process can be performed within the noise transfer time.
As above, it can be understood that the noise reduction effect can be obtained in a frequency band for which a group delay is small and the noise control process can be performed within the noise transfer time, but if there is a frequency band for which a group delay is large and the noise control process cannot be performed within the noise transfer time, a noise increase occurs in the frequency band.
A phenomenon that, while the noise reduction effect can be obtained in a certain frequency band, a noise increase occurs in another frequency band, and the like, is a problem often faced when the noise control is practically used, such as the above-described problem of the group delay. A method for preventing such a disadvantage in a certain frequency band has been conventionally proposed (for example, Japanese Laid-Open Patent Publication No. 5-67948). This method tries to prevent occurrence of a problem by suppressing, in an adaptive filter that performs the noise control, an increase of a coefficient gain for a frequency band in which the problem occurs (a noise increases).
In the following, a conventional method will be described. FIG. 57 shows a fundamental configuration disclosed in Japanese Laid-Open Patent Publication No. 5-67948. In a noise control device shown in FIG. 57, there is a noise source 101 in a casing 102 having an opening at one end thereof. From the noise source 101 toward the opening, a noise detection microphone 103, a sound cancellation speaker 105, and a sound cancellation error detection microphone 104 are placed in this order. In the noise control device shown in FIG. 57, a control circuit using an adaptive digital filter is provided. The adaptive digital filter is formed with a main adaptive digital filter portion and an auxiliary adaptive digital filter portion. The main adaptive digital filter portion is formed with an FIR digital filter 106 and a coefficient controller 108 that is controlled based on an LMS (Least-Mean-Square) algorithm. The auxiliary adaptive digital filter portion is formed with a FIR digital filter 110 and a coefficient controller 111 that is controlled based on the LMS algorithm. The two FIR digital filters 106 and 110 share a coefficient sequence ha(i). Moreover, a filter 109 is connected to a signal input section of the FIR digital filter 110, and a digital filter 107 is connected to the coefficient controller 108.
In the noise control device, a noise detected by the noise detection microphone 103 is converted into a digital signal by an A/D converter 115 via a preamplifier 112, and a noise signal u(n) is generated. Then, the noise signal u(n) is inputted to the digital filters 107 and 109, and the FIR digital filter 106. In the FIR digital filter 106, a control coefficient is calculated based on the predetermined coefficient sequence ha(i), and a noise cancellation signal y(n) is generated. The noise cancellation signal y(n) is converted into an analog signal by a D/A converter 116, and inputted to the sound cancellation speaker 105 via a power amplifier 113. Then, a sound wave outputted from the noise source 101 and a sound wave outputted from the sound cancellation speaker 105 interfere with each other, and thereby the noise outputted from the noise source 101 is canceled. A result of the sound cancellation is detected by the sound cancellation error detection microphone 104, outputted as an error signal e0(n) via a preamplifier 114 and an A/D converter 117, and inputted to the coefficient controller 108. In the coefficient controller 108, the coefficient sequence ha(i) is updated and controlled so as to minimize the inputted error signal e0(n). Here, the digital filter 107 is inserted for correcting the noise signal u(n) to thereby control the coefficient with an increased accuracy.
On the other hand, in the digital filter 109, the noise signal u(n) is inputted and an output signal u1(n) is outputted. Here, the digital filter 109 has high-pass-type frequency characteristics which cause the noise cancellation signal y(n) to have a frequency-characteristics restriction for not outputting an uncontrollable high-frequency sound. The output signal u1(n) outputted from the digital filter 109 is inputted to the FIR digital filter 110. In the FIR digital filter 110, a control coefficient is calculated based on the predetermined coefficient sequence ha(i), and an error signal e1(n) is generated. The coefficient controller 111 updates the coefficient sequence ha(i), based on the output signal u1(n) outputted from the digital filter 109 and the error signal e1(n). In other words, the coefficient sequence ha(i) is updated and controlled in such a manner that when a high-frequency signal passing through the digital filter 109 is inputted to the FIR digital filter 110, the signal is made zero.
In this manner, in the noise control device shown in FIG. 57, the high-frequency signal which is disturbing is cut off, and a sound cancellation control by the adaptive digital filter is performed in a frequency band that allows a stable adaptive operation control.
The noise control device shown in FIG. 57 tries to prevent an occurrence of the problem by suppressing an increase of a coefficient gain for a frequency band in which the problem occurs (a noise increases), by using an adaptive filter that performs a noise control. However, the noise control device shown in FIG. 57 is on the assumption that the noise control process can be performed within the noise transfer time. Therefore, as described with reference to FIGS. 43 to 56, if there is a frequency band for which the noise control process cannot be performed within the noise transfer time, an occurrence of a noise increase in the frequency band cannot be prevented.
In addition, when a configuration and conditions (such as a sampling frequency and the number of taps) of the noise control device are determined, a processing time required by the whole of the device is determined. However, in the conventional method, the only method for performing the noise control process within the noise transfer time in the total processing time (for example, the time τ in FIG. 43) is to increase the length from the noise source to the control point (for example, to increase the noise transfer time T of the noise transfer system of FIG. 43). In this case, as described above, a noise control system as a whole is enlarged, to cause a problem that the size of a product is increased, that a practical use is impossible because a product having an assumed size cannot be obtained, or the like. Moreover, a distance from the noise source to the control point becomes long. Therefore, in a case of FIG. 44 for example, the correlativity between the reference signal a and the error signal b of the signal processor 300 is lowered, and the coefficient of the signal processor 300 cannot be accurately obtained. This causes a problem that a desired noise reduction effect cannot be obtained.